Origin of an unintended increase in carrier density of ternary cation-based amorphous oxide semiconductors

In thin film transistors (TFTs), carrier density in the channel layer is a fundamental intrinsic factor to engineer desirable TFT performance parameters such as the threshold voltage, drain current, and on-to-off ratio. Here, we report on the origin of carrier density modulation in a ternary cation system of InAlZnO (IAZO) and its effect on the TFT performance. Through work function investigations and bandgap analysis, the carrier density of IAZO is found to be increased by >10⁴ times compared to that of unannealed IAZO after low temperature annealing at 200 °C in air. Photoelectron spectroscopic studies reveal that no significant changes were made in dopant concentrations, neither intrinsic (vacancy-based native defect) nor extrinsic (cation substitution) after annealing. From high pressure oxidation with much enhanced reactivity of reaction gases, it is identified that the equilibrium carrier density of IAZO is much higher than those used in typical TFT channel application. The low channel carrier density tends to increase and reach the higher equilibrium carrier density in the absence of kinetic constraints. This notion is further supported by a defect-state transition mechanism. The combinatorial investigations presented herein help understand the origin of the unintentional increase in channel carrier density in amorphous oxides and its effect on the operation of TFTs.